Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P1/00—Details of instruments
  • G01P1/02—Housings
  • G01P1/023—Housings for acceleration measuring devices
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
  • G01D11/00—Component parts of measuring arrangements not specially adapted for a specific variable
  • G01D11/30—Supports specially adapted for an instrument; Supports specially adapted for a set of instruments

Definitions

• the present invention relates to mounting systems for precision transducers and, in particular, to a stress-free mounting system for a transducer such as an accelerometer.
• a precision transducer It is often necessary to isolate a precision transducer from external stress. Such stress may be caused by mechanical distortion of the case or other structure to which the transducer is mounted, or by differential thermal expansion or contraction between the transducer and the case. Isolation from external stress can in principle be achieved by using a compliant mounting system.
• a compliant mounting system will not in general provide precise and stable alignment of the transducer with respect to its case. For many transducers, such alignment is critical for achieving proper operation.
• a compliant mounting system may also result in unwanted mechanical oscillation of the transducer when the case is exposed to vibration.
• An accelerometer is an example of an instrument that must not be allowed to change position or vibrate with respect to its case.
• One prior accelerometer mounting technique has been to connect the accelerometer to the case by means of a metal ring or by means of a structural adhesive such as an epoxy resin. These prior non-compliant mounting techniques result in stress being transmitted to the accelerometer due to differential thermal expansion between the accelerometer and the mounting ring and case. These prior techniques also transmit stress to the accelerometer when the case is subjected to mechanical distortion. Distortion can be induced by mounting the case to a surrounding support, or by differential thermal expansion between the case and the support.
• the present invention provides a mounting system for a precision transducer such as an accelerometer.
• the mounting system is compliant to differential volumetric expansion but rigid against rotation or translation of the transducer with respect to the case.
• the mounting system of the present invention is adapted to support a precision transducer in spaced alignment with a supporting case.
• the mounting system comprises a plurality of mounting elements, each mounting element having first and second ends and a resilient intermediate portion.
• the first end of each mounting element is connected to the transducer, and the second end of each mounting element is connected to the case.
• Adjacent mounting elements are preferably joined to one another by bridge sections to form a mounting ring or band.
• the band has a continuous, inwardly facing mounting surface formed by the first ends and the bridge sections.
• the second ends of adjacent mounting elements are preferably axially separated from the first ends and circumferentially separated from the second ends of the adjacent mounting elements by gaps.
• first end and the second end of each mounting element lie in the same plane and are circumferentially separated from each other by gaps.
• at least the first ends and the bridge sections are composed of a substance that has a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of the transducer.
• the intermediate portion of each mounting means is adapted to provide a low resistance to relative movement between the transducer and the case in a radial direction, and a high resistance to relative movement between the transducer and the case in directions normal to the radial direction. Differential thermal expansion between the transducer and the case therefore does not apply stress to the transducer, or cause misalignment between the transducer and the case.
• FIG. 1 is a perspective view of an accelerometer mounted in a case by the mounting system of the present invention
• FIG's. 2 and 7 are perspective views of the mounting ring of FIG. 1;
• FIG's. 3 and 5 are cross-sectional views showing the connection of a mounting element between the transducer and case of the two embodiments described;
• FIG's. 4 and 6 are side-elevational views of a portion of the mounting ring of the two embodiments described; and
• FIG's. 8 and 9 are fragmentary views of the mounting elements which form the embodiments shown in FIG's. 2 and 7.
• FIG. 1 shows an accelerometer mounted by means of the mounting system of the present invention.
• the accelerometer of FIG. 1 includes a case 12, a transducer 14 and a mounting ring 16.
• the case 12 includes a cylindrical sidewall 18, a bottom wall 20 and a flange 22.
• the flange 22 includes mounting holes 24 that are used to mount the case and accelerometer to a supporting structure.
• the transducer 14 has a generally cylindrical shape and comprises two excitation rings 26 and 27 joined by bellyband 28.
• the transducer 14 is considered to be an accelerometer which is adapted to respond to accelerations along an axial disposed sensitive axis "S" by producing an electrical signal that indicates the direction and magnitude of such acceleration.
• the transducer 14 is mounted to the case 12 at excitation ring 27 by the mounting ring 16. As described below, the mounting ring 16 provides precise and stable alignment of the transducer 14, such that the transducer is not free to undergo translational or rotational movement with respect to the case.
• the mounting ring 16 does permit differential radial or volumetric thermal expansion or contraction between the transducer 14 and the case 12, and also serves to isolate the transducer from stresses that would otherwise result from distortion of the case. Distortion of the case 12 may be caused by mounting the flange 22 on a surface that is not perfectly flat, or by differential thermal expansion between the flange and its support.
• the mounting ring 16 in FIG. 1 is illustrated in greater detail in FIG. 2.
• the mounting ring 16 comprises a plurality of mounting elements 30 (see FIG. 8).
• Each mounting element comprises an upper end 32, a lower end 34 and a resilient intermediate portion or beam 36.
• the upper ends 32 of the mounting elements 30 are attached to the case 12, and the lower ends 34 are attached to the transducer 14.
• the lower ends of each mounting element 30 are joined to the lower ends of adjacent mounting elements by bridge sections 38 (see FIG's. 6 and 8).
• the bridge sections thereby join the mounting elements into a single, generally cylindrical mounting ring 16, as is best illustrated in FIG. 2. It is not required for the practice of the present invention that the mounting elements be joined to one another by bridge sections 28. However, the use of
• SUBgrrn UTE SHEET bridge sections is preferred because it significantly facilitates manufacturing and assembly of the accelerometers.
• FIG's. 3 and 4 illustrate further details of the mounting elements and of the connection between the mounting elements and the transducer 14 and case 12.
• the upper end 32 of each mounting element 30 includes a pad 40 that includes an outwardly facing surface 42.
• This surface 42 preferably has a cylindrical contour that matches the contour of the adjacent inner wall of the sidewall 18 of the case 12.
• the pad 40 is joined to the sidewall 18 by an adhesive layer 44.
• a suitable material for the adhesive layer 44 is a structural adhesive such as an epoxy resin.
• the lower end 34 of each mounting element 30 includes an inwardly projecting flange 46, having a cylindrical inner surface 48 that has a contour that matches the contour of the adjacent outer surface of the transducer 14.
• the cross-sections of bridge sections 38 see FIG.
• FIG. 3 illustrates the use of weld joint 50 to create the bond between the flange 46 and the excitation ring 27.
• the point of attachment of the transducer 14 to the mounting ring 16 is preferably spaced as far as possible from the flange 22 of the case 12, in order to minimize the transmission of stress from the case flange to the transducer.
• the mounting ring 16 is attached to the transducer
• U ⁇ STiTUTE SHEET 14 in such a way that minimal stress is produced when the transducer and mounting ring undergo thermal expansion or contraction.
• This result is achieved by fabricating at least the lower ends 34 and the bridge sections 38 from a material that has a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of the transducer 14, and in the particular embodiment shown in the drawings, of the excitation ring 27.
• An intervening layer of material between the mounting ring and the transducer e.g., an adhesive layer
• any filler metal or brazing material used should be either very thin or have a coefficient of thermal expansion matched to the coefficient of thermal expansion of the transducer and mounting ring.
• the mounting ring 16 is entirely fabricated from a metal identical to the metal forming excitation ring 27, and is welded to the excitation ring part of the transducer 14 without the use of a filler metal. Because of its low coefficient of thermal expansion, Invar, a 36% nickel-iron alloy, is a particularly suitable metal with which to form the excitation ring 27 and the mounting ring 16.
• the upper ends 32 of the mounting elements 30 are preferably not abutting or joined to one another, but are instead spaced apart by gaps 56 (FIG's. 4 and 6). Such gaps eliminate or greatly reduce the high hoop stress that would otherwise occur due to differential thermal expansion or contraction between the mounting ring 16, the adhesive layer, and the case 12. Similar gaps are not required between lower ends 34 of mounting elements 30, because the coefficient
• Each intermediate portion or each beam 36 is dimensioned such that it has a compliant axis oriented in the radial direction (indicated by arrows 52 and 54 of FIG. 3).
• the compliant axis of each beam preferably intersects the axial centerline of the transducer.
• the beam 36 is preferably dimensioned such that it is essentially rigid in directions normal to arrows 52 and 54 (i.e., along the length L of the beam and in the directions into and out of the plane of the drawing in FIG. 3).
• Differential radial or volumetric thermal expansion (or contraction) between the transducer 14, the mounting ring 16 and the case 12 therefore results in differential movement between the transducer and case along the compliant axes of each beam.
• the beams therefore, flex to take up the differential movement without transmitting significant stress to the transducer 14.
• the rigidity of the beams 36 in a direction perpendicular to their compliant axes, results in a mounting system in which the transducer is not free to rotate or to undergo overall translational movement with respect to the case 12. Therefore, in the case of an accelerometer transducer, the sensing axis of the accelerometer is preferably aligned with or in the stiffest or most rigid axis of the beams (i.e., beams in direct compression) . This minimizes frequency response errors and phase shift error due to suspension system mechanical transmissibility.
• each beam 36 The required compliant characteristics of each beam 36 are preferably achieved by making the width W (see FIG. 6) and length L (see FIG. 5) of each beam substantially greater than the thickness T (see FIG. 5) of that beam.
• the width W of each beam 36 must, of course, be limited (with respect to the circumference of
• width-to-thickness ratios between about 10:1 and 20:1 are most suitable, although other ratios may be used, depending on the nature of the transducer and the mounting ring materials.
• One preferred mounting ring 16 comprises twenty-four mounting elements, the beam of each mounting element having a length-to-thickness ratio of about 21:1, and a width-to thickness ratio of about 12:1.
• the distance between adjacent mounting elements i.e., the width of gaps 56
• FIG. 7 illustrates another embodiment of the invention.
• the mounting ring 16 comprises a plurality of mounting elements 30' (see FIG. 9).
• Each mounting element 30' comprises a left end 32', a right end 34' and a resilient intermediate portion or beam 36'.
• the left end 32' of each mounting element 30' is attached to the case 12, and the right end 34' is attached to the transducer
• the bridge sections 38 join the right and left ends of two adjacent mounting elements. Similarly, the bridge sections thereby join the mounting elements into a single, generally cylindrical mounting ring 16, as is best illustrated in FIG. 7.
• FIG's. 5 and 6 illustrate further details of the mounting elements 30' and of the means by which the mounting elements are connected to the transducer and case.
• the right end 34* and the left end 32' of each mounting element 30' lie generally in the same plane (i.e., a horizontal plane perpendicular to the cylindrical axis of the transducer 14).
• each end 32' and 34' lie generally in the same plane (i.e., a horizontal plane perpendicular to the cylindrical axis of the transducer 14).
• each intermediate portion or beam 36' is attached to either the case 12 or the transducer 14, but not both. This feature, combined with the preferred compliant characteristics of the beam 36', reduces the amount of the external stress applied to the transducer.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Fluid Pressure (AREA)

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Publications (1)

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Family Applications (1)

Country Status (5)

Cited By (3)

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Citations (4)

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• 1987
  • 1987-06-25 EP EP19870904616 patent/EP0272316B1/en not_active Expired
  • 1987-06-25 WO PCT/US1987/001572 patent/WO1988000348A1/en not_active Ceased
  • 1987-06-25 DE DE19873783126 patent/DE3783126T2/de not_active Expired - Fee Related
  • 1987-06-25 JP JP62504283A patent/JPS63503572A/ja active Granted
  • 1987-06-26 IL IL8300887A patent/IL83008A/en not_active IP Right Cessation

Patent Citations (4)

Non-Patent Citations (1)

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